1. Field of the Invention
The present invention relates to an optical information recording and reproducing apparatus which has a semiconductor laser as a light source, directs a laser beam of the semiconductor laser to an optical information recording medium by an optical system to record information, reproduce information recorded on the medium and/or erasing the information recorded on the medium.
2. Related Background Art
Commercialization, research and development of the optical information recording and reproducing apparatus such as compact disk, rewritable electronic file system and optical disk system which uss an erasable opto-magnetic material or phase transition type material have recently been actively being conducted. An optical card system for recording and reproducing information on and from a card-like optical recording medium (hereinafter referred to as an optical card) has been recently being noticed. The optical card is characterized by that it is easy to carry and that it has a larger information capacity than a disk (in which a center area is not used for recording information).
The surface of the optical recording medium is scanned by a light beam modulated with record information and focused into a fine spot so that information is recorded thereon as optically detectable record pit train (information track). In order to exactly record information without trouble of crossing of information tracks, it is necessary to control the irradiation position of the light beam in a direction normal to the scan direction in a plane of the optical card (This is hereinafter called an auto-tracking AT). In order to irradiate the light beam as a stable fine spot irrespective of warp or mechanical tolerance of the optical card, it is necessary to control the irradiation position in a normal direction to the plane of the optical card (This is called an auto-focusing AF).
FIG. 1 shows an optical head used in a prior art optical information recording and reproducing apparatus which uses an optical card.
In FIG. 1, a light beam emitted from a light source 1 such as a semiconductor laser is collimated by a collimater lens 2 and split into three beams by a diffraction grating 3. A 0-order diffraction light is used to record and reproduce information and detect an AF error signal, and .+-.1 - order diffraction lights are used to detect an AT error signal. The split light beams are reflected by a beam splitter 4 and directed to an optical card 6 by an objective lens 5 to form three beam spots S.sub.1, S.sub.2 and S.sub.3. The light beams reflected by the optical card 6 again pass through the objective lens 5 and the beam splitter 4 so that they are separated from the incident beams. The reflected beams are reflected by a mirror 7 and focused by a sensor lens 8 and a cylindrical lens 9 and directed to photo-sensors 10.sub.1, 10.sub.2 and 10.sub.3, which are arranged to receive the lights from the beam spots S.sub.1, S.sub.2 and S.sub.3, respectively.
A photo-sensing plane of the photo-sensor 10.sub.2 is divided into four sections as shown in FIG. 2, and sums of detection signals of a pair of diagonally arranged photo-sensing sections are differentiated to detect an astigmatism introduced by the cylindrical lens 9 and produce a focusing signal by a known principle of astigmatism method. When information is to be reproduced, the photo-sensor 10.sub.2 produces a reproduced signal. The signals detected by the photo-sensors 10.sub.1 and 10.sub.3 are differentiated by a differential amplifier 11 which produces a tracking signal ST at a terminal 12. The tracking signal ST is fed back to a lens actuator through a circuit not shown, and an objective lens 5 is moved normally to an optical axis to effect the auto-tracking.
The optical card 6 is reciprocally moved normally to an arrow R, that is, normally to the plane of drawing of FIG. 1 by a drive mechanism not shown so that the light spots S.sub.1, S.sub.2 and S.sub.3 scan the optical card 6. The optical head 19 including the optical system is movable in the direction R to permit track access.
In the prior art optical head, the objective lens 5 receives a light beam which is of substantially equal size to an aperture of the lens 5, as shown in U.S. Pat. No. 3,957,630 in order to reduce a proportion of that light beam diffracted by the optical information pattern (pits) on the recording medium which again passes through the objective lens and reaches the photo-detector so that a contrast of the photoelectrically detected signal is enhanced.
FIG. 3 illustrates the method described above. In FIG. 3, a light beam 100 applied to the objective lens 5 has a light beam diameter which is substantially equal to the aperture of the objective lens 5, and it is focused onto an information recording medium plane 6.sub.2 of the optical card 6 by the objective lens 5 to form three beam spots S.sub.1, S.sub.2 and S.sub.3. A transparent protective layer 6.sub.1 is formed on the information recording medium plane 6.sub.2. Of the three beam spots, the beam spot S.sub.2 is considered. If there is a pit 6.sub.3 on the information recording medium plane 6.sub.2, the reflected light beam produces a diffraction light 200 which is again directed to the objective lens 5. The light beams which pass through the objective lens 5 and are directed to the photo-sensors not shown are those light beams which are within the aperture of the objective lens 5, and the diffracted light 200 outside the aperture do not pass through the objective lens 5. When there is no pit 6.sub.3, no diffracted light 200 is produced and almost all of the reflected light beams pass through the objective lens 5. Accordingly, the smaller the amount of the diffracted lights 200 which pass through the objective lens 5 is, that is, the smaller the aperture of the objective lens 5 is, (more precisely, the smaller the NA of the objective lens 5 is), the larger is the contrast of the photo-electrically produced signal.
The optical card 6 is usually made of a flexible material such as polycarbonate, vinyl chloride resin or acrylic resin, the optical card is warped or bent after long time use if, for example, mechanical pressure or heat is applied during storage. If the deformed optical card is used to record or reproduce information, various problems arise. These are explained below.
FIGS. 4A and 4B illustrate the problems and they show a portion of the apparatus of FIG. 1. The like elements to those shown in FIGS. 1 and 3 are designated by the like numerals. As shown in FIG. 4A, if the information recording medium plane 6.sub.2 of the optical card 6 is normal to the center light beam of the 0-order diffracted light, the reflected lights of the 0-order diffracted light and the .+-.1-order diffracted lights pass through the objective lens 5. If the recording medium plane 6.sub.2 inclines by an angle .theta. is shown by 6.sub.2 ' in FIG. 4B, the reflected light of the -1-order diffracted light on one side does not pass through the objective lens 5. Thus, the amount of light applied to the photo-sensors 10.sub.1 and 10.sub.3 shown in FIG. 5 changes with the inclination of the medium plane 6.sub.2 and an error signal which erroneously indicates tracking error is produced. Accordingly, it is difficult to attain correct auto-tracking because of the inclination of the recording medium plane 6.sub.2.